The present invention relates to an improvement of a toluene diisocyanate (TDI) recovery and purification process which uses a dividing wall column for the final purification of the TDI product. The process of the present invention benefits from the ability to achieve a higher TDI purity.
The field of art to which this invention pertains is a process for the purification of toluene diisocyanate (TDI) mixtures. TDI mixtures are generally produced by reacting toluene with nitric acid to yield dinitrotoluene (DNT), hydrogenating the resultant dinitrotoluene (DNT) to yield toluene diamine (TDA) and reacting the toluene diamine (TDA) with phosgene to give toluene diisocyanate (TDI). Toluene diisocyanate (TDI) is a commercial by available material which is particularly useful in the preparation of polyurethanes, polyureas and polyisocyanurate polymers, especially foamed polymers.
DE-A1-3736988 teaches that organic mono- or polyisocyanates are continuously prepared by reacting the corresponding mono- or poly-amine dissolved in an inert organic solvent with phosgene also dissolved in an inert organic solvent at a temperature under 150° C. The amine and phosgene solutions are combined and allowed to pass through one or more reaction columns connected in series and having at least 10 chambers in total separated from each other by perforated plates, the holes of which preferably have a maximum diameter of 20 mm.
EP-A1-570799 teaches that production of aromatic diisocyanates is effected by reaction of diamines and phosgene. The phosgene and diamine are at or above the boiling temperature of the diamine and the reaction has an average contact time of 0.5–5 seconds. The mixture is continuously passed through a cylindrical reaction space at 200–600° C. to complete the reaction with avoidance of back mixing. The gas mixture is then cooled to condense the diisocyanates, with the temperature being maintained above the decomposition temperature of carbamic acid chlorides corresponding to the diamines used. Uncondensed diisocyanate is washed out of the gas mixture with an inert solvent, and the inert solvent is recovered by distillation.
The Polyurethane Handbook (Oertel, G. (Editor), Polyurethane Handbook, Munich, Germany: Hanser Publishers, 1985, pp 62–73) gives a description of a state of the art phosgenation and distillation process for the production of toluene diisocyanate. In the distillation process, the solvent is completely removed from the crude TDI mixture as the top product from a solvent column, with this solvent being returned to the phosgenation or to the excess phosgene recovery. The remaining crude isocyanate bottoms stream from the solvent column is sent to a pre-flasher where two products are achieved: an isocyanate-rich overhead product and a residue-enriched bottoms stream which is fed to the residue removal. In the residue removal, the volatiles are then removed from this residue-enriched stream and condensed. The condensed volatiles from residue removal together with the condensed overhead stream from the pre-evaporation are then combined and fed to an isocyanate column. In the isocyanate column, the product isocyanate is recovered as a top stream while a high-boiler enriched bottoms stream is returned to the pre-evaporation step. This process is limited by the fact that the complete solvent removal is performed in one solvent column. While it is known that TDI yields are negatively affected by higher temperatures, complete solvent removal necessitates operating under relatively low pressures to achieve sump temperatures low enough to prevent a loss of yield, thus necessitating a large column. Moreover, the long residence-time of isocyanate together with residue in heating zones can lead to a higher rate of residue formation. Finally, condensation of the overhead stream from the pre-evaporation before feeding to the isocyanate column is energy inefficient.
In Industrielle Aromatenchemie (Franck H.-G. and Stadelhofer J., Industrielle Aromatenchemie. Berlin, Germany: Springer Verlag, 1987, p 253), a second state-of-the-art process is described. In the described process, the crude TDI-solvent mixture is fed to a two-step pre-evaporation step resulting in a low-boiling overhead vapor product and solvent-free residue-enriched bottoms product which is fed to the residue removal. In the residue removal process, the volatiles are then removed from this residue-enriched stream and condensed. The overhead product from the pre-evaporation is fed to a solvent column. In the solvent column, the solvent is completely removed as the top product, with the solvent being returned to the phosgenation or to the excess phosgene recovery. The remaining crude isocyanate bottoms stream from the solvent column is fed along with the condensed volatiles from residue removal to an isocyanate column. In the isocyanate column, the product isocyanate is recovered as a top stream while a high-boiler (polymeric isocyanate and hydrolyzable chloride compounds (HCC), and other non-volatiles ) enriched bottoms stream is returned to the pre-evaporation step. This process is also limited by the fact that the complete solvent removal must be performed in one solvent column. As in the process described in the Polyurethane Handbook, complete solvent removal necessitates operating under relatively low pressures to achieve sump temperatures low enough to prevent a loss of yield, resulting in a large solvent column. However, this process, in comparison with the former process achieves a reduced residence-time of isocyanate together with residue in heating zones possibly leading to a lower rate of residue formation. Moreover, because there is no needless condensation of a vapor feed to the isocyanate column, this process will be more energy efficient.
From Chem System's PERP Reportfor TDI/MDI (Chem Systems, Process Evaluation Research Planning TDI/MDI 98/99S8. Tarrytown, N.Y., USA: Chem Systems, 1999, pp 27–32) for TDI/MDI it can be learned, that the fractionation of a crude TDI distillation feed product can be completed in the following manner. Normally, the liquid product from the de-phosgenation stage is sent to a pre-evaporator which produces a residue-rich liquid-phase as a bottom product and a vapor-phase product containing mainly solvent and isocyanate as an overhead product. The bottom product from the pre-evaporation is sent to a process for the removal of volatile compounds from the reaction residues (residue removal). The volatile components removed in the residue removal stage as well as the vapor-phase product from the pre-evaporator are sent to a solvent column, where an initial separation of the isocyanate from solvent is completed as well as the removal of any remaining phosgene. The resulting products are a phosgene-enriched top product, a relatively pure solvent stream as an intermediate product and an isocyanate-enriched bottoms product. The phosgene stream is then returned to the de-phosgenation process or to the excess phosgene recovery process. The solvent product is then used in the phosgenation section as well as in the excess phosgene recovery. The bottoms isocyanate-rich product is then sent to a second solvent removal column where the remainder of the solvent is removed. The top solvent product from this step, when relatively pure, can be used in phosgenation or excess phosgene recovery or can be returned to the primary solvent removal step. The final solvent-free bottoms isocyanate product is sent to an isocyanate column, resulting in an isocyanate top product and a residue and hydrolyzable chloride compound (HCC) enriched-bottom stream which is returned to the pre-evaporation or to the residue-removal stages. This process like the process described in Industrielle Aromatenchemie, in comparison with the process described in the Polyurethane Handbook achieves a reduced residence-time of isocyanate together with residue in heating zones possibly leading to a lower rate of residue formation. Additionally, like the process described in Industrielle Aromatenchemie, because there is no needless condensation of a vapor feed to the isocyanate column, this process will be more energy efficient than the process disclosed in the Polyurethane Handbook. It holds the additional advantage, that the solvent removal is completed in two-steps. By taking advantage of the fact that the solvent has a lower boiling point than the isocyanate, the majority of the solvent can be removed under higher pressure, thereby reducing the necessary investment cost for the solvent removal. Additionally, the use of two solvent removal steps adds to the flexibility of operation. However, the presence of a third column adds more complexity to the process.
In fractionation, it is sometimes desirable to separate a multi-component feed stream into a number of streams containing various fractions of desirable components in the product streams. For the case of one feed stream and two product streams, the separation can be accomplished by distillate and bottoms product draw. Further separation can be accomplished by repeating the two-product stream process to either the distillate or the bottoms streams. However, the introduction of additional columns will require a corresponding number of reboilers and condensers. That requirement, in turn, requires additional operating costs as the condensing and the reboiling process is being repeated. Numerous references can be found in prior art documenting efforts to lower both capital and operating costs in the separation of several fractions from a multi-component feed stream The benchmark of the lowest energy consumption has been set by the old and well-known PETLYUK system (Agrawal, R and Fidkowski, Z, “Are Thermally Coupled Distillation Columns Always Thermodynamically More Efficient for Ternary Distillations?”, Industrial & Engineering Chemistry Research, 1998, 37, pp 3444–3454). In this configuration, a pre-fractionation column separates the feed into two streams using a split vapor stream from the main column's stripping section and a split liquid stream from the main column's rectifying section. The resulting vapor and liquid streams exiting from the pre-fractionation column are richer in light and heavy components respectively. These two semi-processed streams are then fed back to the main column. This configuration provides an advantage allowing the main fractionation column to enhance the purity of the side stream draw. In turn, the main fractionation column also provides the stripping section and the rectifying section with better quality feeds. The combined effect is a very efficient use of vapor/liquid traffic to yield three product streams. U.S. Pat. No. 2,471,134 teaches an improvement of the Petyluk process with a proposal to combine the pre-fractionation and main columns into one fractionation unit by erecting a partition along the center part of a column. The column is equipped with one overhead condenser and one bottom reboiler.
The dividing-wall distillation column described in U.S. Pat. No. 2,471,134 is a vertical column fractionating tower, equipped with reboiler and condenser, which is divided into four distinct column sections by the use of a center partition in the intermediate part of the column. These sections are a common bottom (stripping) and top (rectifying) sections, and the pre-fractionation and main fractionation sections in the intermediate part of the column are separated by a dividing-wall. The multi-component mixture is fed to the pre-fractionation section, the overhead product is taken from the common rectifying section, a bottoms product is taken from the common stripping section, and the intermediate product stream is taken as a side-product from the main fractionation section. One significant advantage of the use of a dividing-wall distillation column is the fact that the sidedraw product can be obtained from the dividing wall distillation column with lower-concentration of low-boiling impurities than that of a side product which is obtained from a simple sidedraw product column.
This dividing-wall distillation column is effective in overcoming the hydraulic limitations in the PETLYUK system. At the same time, it reduces capital costs by having only one common shell. The dividing-wall distillation column disclosed in U.S. Pat. No. 2,471,134 has found applications in several processes.